Limited-area cathode



O 1951 H. D. SHEKELS 7 LIMITED-AREA CATHODE Filed Aug. 16, 1949 INVENTOR HOWARD D. SH EEKELS Patented Oct. 2, 1951 2,570,165 LIMITED-AREA ce'rnonr,

Howard D. Shekels, St. Paul, Minn., assignor to Radio Corporation of America, a corporation of Delaware Application August 16, 1949, Serial No. 110,575

1 Claim. 1

The present invention relates to an improved gun structure for an electron beam device and to a process for making it. More particularly it relates to an improved gun structure wherein the cathode includes a limited-area or punctiform emission surface supported in exact alignment with an aperture of the control grid and to a process for making it. i

As is known, it is desirable for certain electron beam tubes, such as ones used in television for analyzing objects and reconstituting images thereof, that in each tube substantially all of the electrons supplied to its beam should be drawn from a very small and precisely shaped area of its cathode to make it possible to obtain sharp focusing of the beam on the target area.

One way of devising an appropriate gun structure is disclosed in U. S. Patent 2,131,204. Ac-

- cording to this patent the cathode is formed with a limited-area emissive surface, i. e., a small mass of emissive material is deposited within a small aperture accurately formed in the center of a non-emissive area of the cathode, which is much larger than the aperture and faces toward the tube target. For example, the small aperture may be formed in the front end of a thimbleshaped outer element of the cathode which serves as a shield and is made of material (such as nickel) having low emissivity. A cathode of this kind is first constructed as a sub-assembly which must then be mounted in the gun so that the small emissive mass is in exact alignment with an aperture in another electrode such as a I control grid. If exact alignment is not attained, that other element the grid) will have a tendency to intercept some of the emitted electrons no matter how precisely the punctiform emission surface may have been formed. This will cause the grid to draw current when it is driven positive, and therefore it will decrease the input impedance which the beam device presents to a circuit for modulating the control grid. In addition, it will distort the cross-sectional shape of the beam by masking the current coming from one side of the accurately-formed emissive portion of the cathode. Yet, in the practical matter of mass producing guns of this type with machines which are only ordinarily precise and with workers who have only average skill, it is almost impossible to obtain exact alignments of the cathodes and grids of the finished guns without tolerating a very high percentage of manufacturing shrinkage.

An alternative practice is known by which it is possible to obtain some of the desirable performance characteristics of guns having limitedarea, or punctiform, emission surfaces properly aligned with the control grid aperture. It is to construct the cathode with a relatively large emissive surface and to utilize the grid to mask all of that surface except the portion under the grid aperture so that the aperture will determine the cross-sectional size and shape of the beam. This type of construction does not entail any need for precise alignment of the cathode and control grid in assembling the gun since the emissive surface of the cathode is so large that, no matter how inaccurately these parts may be assembled together, the control grid aperture will register with some part of it.

However, this construction has a number of disadvantages of its own. For one thing, the grid will draw considerable current whenever it is driven positive. This is due to the fact that it will collect electrons from a very large emitting surface, i. e., from all of the emitting surface of the underlying cathode except for the very small portion thereof which lies directly beneath the grid aperture and provides the beam electrons. In addition, the size of the beam cross-section will vary depending upon the potential difference between the grid and the cathode. This is partly due to the combined effect of: (1) the fact that the effective size of the grid aperture varies with its potential (the more negative the grid is the smaller the effective aperture will be); and (2) the fact that in this type of gun the cross-section of the beam is not lim it d by the size of the emissive area of the cathode. In addition, it is partly due to the further fact that when the grid is near to the cathode potential, or is positive with respect thereto, many of the electrons which pass through the aperture will move with radial components. This will produce on the screen of 'a picture tube the effect known as blooming.

For these reasons, in the art of electron beam devices a need has existed for some time for a gun structure in which the cathode has a limitedarea, or punctiform, emission surface and in which the emission surface is exactly aligned with the aperture of the control grid independently of the precision with which the cathode and the control grid are assembled together. Besides making it possible to obtain sharp focusing of the beam on the target area, such a structure offers a number of other advantages. Since the emitting area does not have to be limited (electrically) by the grid aperture, that aperture may be made large enough so that the electron optical field aberration, which is pronounced at the edge of the aperture, can be kept away from the electron beam. Moreover, the. use ofa lar aperture permits the grid-to-cathode spacing to be increased and renders this spacing less critical than usual while at-the same time preserving the same control characteristics (beam cut-off and grid bias). In addition if desired this structure permits designs which afford a sharper beam current cut-oil, this being a characteristic which is frequently very desirable.

It is an object of the present invention to devise an improved gun structure for an electron beam device wherein the cathode will have a limitedarea, or punctiform, emission surface which is in exact alignment with the aperture of the control grid independently of the accuracy with which the cathode and the control grid are assembled together.

It is a further object of the present invention to devise an improved electron gun having a limited-area emission surface which is smaller than the aperture of the control grid and is ment with the aperture of the control grid without necessitating the usevof unusually precise machinery or extraordinary skill.

Other objects, features and advantages of this invention will be apparent to those skilled in the art from the following detailed description of the invention and from the drawing in which:

Fig. 1 represents an electron beam device embodying the improved gun disclosed herein;

Fig. 2 represents a grid-cathode assembly embodying the present invention and illustrates the method of forming its limitedaarea emission surface according to the present invention; and

Fig. 3 further illustrates the present method for forming a limited-area cathode.

In general, the improved electron gun structure according to the present invention comprises, as its distinctive feature, a grid-cathode subassembly including a control grid having a central aperture, a cathode having a non-emissive surface underlying the control grid, and a punctiform emissive coating carried on a limited portion of the non-emissive surface which lies end of the outer neck l3 of the double-walled envelope l2. Assembly II is supported partly on cathode tabs, one of which, I5, is shown in Fig. 1, and partly by being combined with other elements of the electron gun of the kinescope to form a unitary structure which has its right-hand end centered in the outer neck l3 by a-spacer l1. Adjacent to the grid-cathode assembly N there is a second grid electrode l9. It is supported by a number of radially disposed supporting members each of which has its inner end welded to the second grid and its outer end fused into one of a number of elongated glass beads 2| which extend inside and lengthwise of the neck I 3. similar supporting members are used for attaching the grid-cathode assembly to the elongated beads 2|. Adjacent to the side of the second grid electrode l9 which faces away from the grid-cathode assembly I is a first accelerating anode 24. It is supported in this position by the elongated beads 2| to which it, like other elements described above, is connected by a number of radial supporting elements. The spacer I1 is carried on the first accelerating anode 24 and acts to center it within the inner walls of the neck l3.

The double-walled envelope |2 includes an inner neck l4 having an open end which faces toward the electron-gun end of the tube and is capped with a focusing-electrode l6. The focusing-electrode l6 may be supported by the inner neck I in the manner disclosed in co-pending application Serial No. 96,017. The final anode directly .beneath the center Of the grid aperture and is smaller than the aperture. And, in general, the method for making the gun comprises forming all of the grid-cathode sub-assembly except for the emissive coating and then spraying the coating onto the non-emissive surface of the cathode through the grid aperture while guiding the spray thereto through a duct in a spray-mask which is positioned for properly guiding the spray by being inserted into the aperture itself.

Fig. 1 shows an electron beam device M in consists of a layer of. conductive material I! which is deposited on the inner surfaces of the inner neck I4 and of the inner side walls of bulb portion of the envelope I2. At one end of the bulb portion there is a relatively flat wall 20,

which carries a fluorescent screen 22 on its inside surface.

Fig. 2 shows in detail a suitable form of construction for the grid-cathode assembly I This assembly comprises an outer cylinder 30 to the outside of which are attached the radially disposed supporting members for connecting the assembly to the elongated beads l2. At one end of the outer cylinder 30 there is welded a griddisc 3|, which has a very small central aperture 32 which may be of the order of .036" in diameter. Welded around the inner edge of one end of the cylinder 30 directly beneath the grid-disc 3| there is an annular grid-cathode spacer 33 having a shoulder 34 against which a'supp'orting means for the cathode may be made to abut. The spacer 33 serves to fix the distance between the top surface of the cathode and the bottom surface of the grid-disc 3|. The supporting means for the cathode is a ceramic spacer 35 formed as a washer. i. e., it has a circular perimeter and a circular perforation concentric therewith. The washer is of such size that it fits closely within the cylinder 30 and its central perforation is of appropriate size to receive a cathode-thimble 36. The cathode-thimble is formed with a pair of shoulders 31 between which the ceramic spacer is held captive. The upper side of the ceramic spacer 35'abuts against the shoulder 34 of the cathode-grid spacer 33 and it is held in this position by a grid shield 38. To this end the shield 33 is welded to the inside of the cylinder 30 and abuts against the lower side of the spacer 35. A limited-area, or punctiform, emissive coating 33 is deposited on the top surface of the cathode, i. e., on the closed end of the cathode-thimble 33, directly beneath the grid aperture 32. According to the present invention, the coating 39 is formed thereon as disclosed below.

In Fig. 2 a spray mask 49 is shown lifted away from a position of engagement with the gridcathode assembly II in which it is utilized for forming the limited-area coating 39. Fig. 3, on the other hand, shows it in its position of engagement. Spray mask 45 includes a dish-shaped portion 4| and, at the center thereof, a very small neck 42 which has a cylindrical external surface of appropriate size to fit snugly within the aperture 32 and, concentric therewith, an open duct 43 through which emissive material may be sprayed and guided to form the limited-area coating 39.

When used for this purpose the spray mask is placed on top of the grid-cathode assembly II with the neck 42 extending through the aperture 32.

The spray mask 40 may be formed in any suitable manner. For example, it may be stamped of thin metal so that the structure of the neck 42 consists of an extrusion of a portion thereof at the center of the dish 4|. However, it may be turned on a lathe if it is desired to form it more precisely.

In practicing the invention, it will be advantageous to clean out the duct 43 between each use of the spray mask 40, or at least after it has been used for but a limited number of times.

By following this process for applying the emissive coating to the cathode-thimble, the coating will always be in exact alignment with the aperture 32 irrespective of the accuracy with which component parts of the grid-cathode assembly may have been put together. In addition this coating will aways be smaller in size than this aperture. Since grid-current is usually drawn only from any portions of the emissive coating of the cathode which may directly underlie the solid part of the grid-disc (and, when the grid is driven quite positive, only from the edges of the portion which lies directly beneath the grid aperture), and since the limited area described herein is smaller than the grid aperture 32, the amount of grid current will never be very substantial during normal operation. Moreover, since the crosssectional size of the beam will be limited by the small size of the emissive coating 39 rather than by the aperture 32, variations in the potential 8 applied to the grid will have far less effect upon the thickness of the beam and therefore will act primarily only to intensity-modulate it.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What I claim is:

A sub-assembly for an electron gun comprising a cup-shaped grid having an aperture at the centerof its'closed end to pass electrons, rigidly supported within the grid a thimble-like cathode the closed end of which has a relatively flat nonemissive surface registering with said aperture, a spacer accurately fixing the distance between said non-emissive surface and the under side 01' said aperture, and a coating of emissive material formed on a central portion of said non-emissive surface beneath said aperture, the coating being of smaller area than the aperture and in exact alignment with it independently of the degree of coaxiality between said grid and cathode.

HOWARD D. SHEKELS.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 1,256,599 Schoop Feb. 19, 1918 2,107,784 Gardner Feb. 8, 1938 2,125,418 Benjamin et al Aug. 2, 1938 2,131,204 Waldschmidt Sept. 27, 1938 2,141,414 Schlesinger Dec. 27, 1938 2,146,365 Batchelor Feb. 7, 1939 2,195,914 Baler Apr. 2, 1940 2,244,358 Ewald June 3, 1941 2,272,165 Varian et al. Feb. 3, 1942 2,310,811 Schantl et al. Feb. 9, 1943 2,496,825 Szegho Feb. 7, 1950 2,507,812 Phipps May 16, 1950 2,507,979 Kelar May 16, 1950 FOREIGN PATENTS Number Country Date 107,623 Australia June 15, 1939 

